When Something's Wrong With Your Fish
You develop a sixth sense for it after a while. You glance at the tank during your morning coffee and something just feels off. Maybe a fish is hiding that's usually front and center. Maybe their color looks washed out. Maybe they're rubbing against a rock in a way that seems deliberate.
Fish disease is one of those topics that nobody thinks about until they're staring at a sick fish and frantically searching the internet at midnight. The good news is that most common aquarium diseases are treatable if caught early. The bad news is that by the time symptoms are visible, the disease has usually been progressing for days. That's why regular observation is so important — the sooner you notice something, the better the outcome.
I've dealt with every disease on this list at some point over 15 years of fishkeeping. Here's what actually works, what doesn't, and when to know the difference.
Ich (White Spot Disease)
If there's one disease every fishkeeper encounters eventually, it's ich. Ichthyophthirius multifiliis is a protozoan parasite that appears as small white dots on the body and fins, resembling grains of salt. Infected fish often rub against objects (called flashing), clamp their fins, and may become lethargic or lose appetite.
What Causes It
Ich parasites are present in most aquariums at low levels. They become a problem when fish are stressed — by temperature changes, poor water quality, new additions that weren't quarantined, or transport stress. A healthy fish with a strong immune system can usually fight off low-level ich exposure. A stressed fish can't.
How to Treat It
The white dots you see are actually parasites embedded in the fish's skin, protected by a membrane. Medication can only kill ich during its free-swimming stage, after it drops off the fish and before it reattaches. This is why treatment takes time.
- Raise the temperature gradually to 82-86 F over 24 to 48 hours. This accelerates the parasite's lifecycle, forcing it through the vulnerable free-swimming stage faster.
- Add aquarium salt at 1 tablespoon per 5 gallons (not all fish tolerate salt — corydoras and some scaleless fish are sensitive, so research your specific species first).
- If salt isn't appropriate for your tank inhabitants, use an ich medication containing malachite green or a combination of malachite green and formalin. Follow the dosage instructions exactly.
- Continue treatment for at least 3 days after the last visible white dot disappears. The parasites you can't see are still cycling.
Fin Rot
Fin rot starts subtly — edges of the fins that look slightly uneven or discolored. Left untreated, it progresses to ragged, receding fins with white, red, or black edges. In severe cases, it can eat away the entire fin down to the body.
What Causes It
Almost always poor water quality. Elevated ammonia or nitrite damages fin tissue, and opportunistic bacteria move in. Overcrowded tanks, infrequent water changes, and overfeeding are the usual underlying factors. Physical damage from sharp decorations or aggressive tank mates can also create an entry point for infection.
How to Treat It
Mild fin rot is one of the easiest diseases to treat because the cause is usually straightforward:
- Test your water immediately. Address any ammonia, nitrite, or high nitrate readings.
- Perform daily 25 percent water changes for 7 to 10 days.
- Remove any sharp decorations that could be causing physical damage.
- For mild cases, clean water alone is often enough. Fins will regrow over several weeks.
- For moderate to severe cases, treat with an antibacterial medication. Kanaplex (kanamycin) and API Erythromycin are both effective. Always complete the full course of medication.
Dropsy
Dropsy is the one that keeps experienced fishkeepers up at night. A fish with dropsy develops a swollen, bloated body with scales that stand out from the body like a pinecone. It's a heartbreaking sight, and unfortunately, the prognosis is usually poor by the time it's visually apparent.
What It Actually Is
Dropsy isn't a disease itself — it's a symptom of internal organ failure, usually kidney failure. Fluid accumulates in the body cavity, creating the characteristic swelling and raised scales. The underlying cause can be bacterial infection, viral infection, parasites, or organ damage from chronic poor water conditions.
Treatment Options
Be honest with yourself about the severity before starting treatment. If scales are severely raised all around the body, the internal damage is likely too extensive for recovery.
- Isolate the affected fish in a hospital tank immediately to prevent potential spread and to treat without affecting the main tank.
- Add Epsom salt (not aquarium salt) at 1 tablespoon per 5 gallons to help draw out excess fluid.
- Treat with a broad-spectrum antibiotic food like Kanaplex mixed into gel food if the fish is still eating.
- Keep water pristine in the hospital tank with daily water changes.
- If there's no improvement in 5 to 7 days, or if the fish stops eating and becomes listless, euthanasia with clove oil may be the most humane option.
Velvet (Gold Dust Disease)
Velvet is caused by the parasite Oodinium, which creates a fine, gold or rust-colored dusting on the fish's body. It's harder to spot than ich — the individual parasites are much smaller. The easiest way to check is to shine a flashlight on the fish from the side in a darkened room. Velvet will show as a distinct metallic sheen.
Symptoms
Fish with velvet often clamp their fins, flash against objects, breathe rapidly, and lose color. They may hover near the surface gasping. Velvet attacks the gills as well as the skin, making it potentially more dangerous than ich if left untreated.
Treatment
- Darken the tank completely. Velvet parasites are partially photosynthetic and rely on light for energy. Covering the tank with a blanket or towel weakens them significantly.
- Raise the temperature to 82-84 F to accelerate the parasite lifecycle.
- Treat with a copper-based medication following the manufacturer's directions precisely. Copper is effective against velvet but is lethal to invertebrates (snails, shrimp), so remove them before treatment.
- Continue treatment and darkness for at least 10 to 14 days to ensure all life stages of the parasite are eliminated.
Columnaris (Cotton Mouth Disease)
Often confused with a fungal infection because of its white, cottony appearance, columnaris is actually a bacterial disease caused by Flavobacterium columnare. It appears as white or gray patches on the mouth, body, or fins, sometimes with a stringy, thread-like texture. It can progress rapidly and is often fatal within 48 to 72 hours in its acute form.
Treatment
Speed is critical with columnaris. Unlike many fish diseases where you can take a wait-and-see approach with clean water, columnaris demands immediate action.
- Lower the temperature to 75 F or below. Unlike ich and velvet, columnaris bacteria thrive in warm water. Lowering the temperature slows bacterial reproduction.
- Treat with Kanaplex (kanamycin) or a combination of Furan-2 and Kanaplex for severe cases.
- Add aquarium salt at 1 tablespoon per 5 gallons to create an inhospitable environment for the bacteria.
- Improve water quality and reduce stressors. Columnaris almost always strikes fish that are already stressed by poor conditions.
Swim Bladder Disorder
When a fish is floating sideways, stuck at the surface, or sinking and unable to swim up, swim bladder disorder is the likely culprit. The swim bladder is an internal gas-filled organ that controls buoyancy. When it malfunctions, the fish loses the ability to maintain normal position in the water.
Common Causes
- Overfeeding and constipation (the most frequent cause by far)
- Swallowed air from surface feeding
- Bacterial infection of the swim bladder
- Physical injury
- Genetic deformity, especially in heavily inbred fancy goldfish
Treatment
- Fast the fish for 2 to 3 days. This is the first thing to try and often resolves the issue if constipation is the cause.
- After fasting, offer a small piece of blanched, deshelled pea. The fiber content helps move things along.
- If fasting doesn't help, Epsom salt baths (1 tablespoon per gallon in a separate container for 15 minutes) can reduce swelling around the swim bladder.
- For suspected bacterial infections, treat with an antibiotic like Kanaplex in a hospital tank.
Prevention: The Best Medicine
Most fish diseases are preventable. That's not just a platitude — it's the honest truth of the hobby. Here's what consistent prevention looks like:
- Quarantine all new fish for 2 to 4 weeks in a separate tank before adding them to your main display. This single step prevents the vast majority of disease introductions.
- Maintain excellent water quality through regular testing and weekly water changes. Healthy water means healthy fish with strong immune systems.
- Don't overstock. More fish means more waste, more stress, and more disease risk.
- Don't overfeed. Feed only what fish can consume in 2 to 3 minutes. Uneaten food fouls the water and creates conditions for bacterial blooms.
- Watch your fish daily. Spend a few minutes each day just observing behavior. You'll learn what normal looks like, which makes it much easier to spot early warning signs.
Fish disease is stressful, but it's manageable when you know what you're dealing with. Keep a basic fish medicine cabinet stocked with Kanaplex, ich medication, Epsom salt, and aquarium salt, and you'll be prepared for the most common problems before they become emergencies.